1. Field of the Invention
The present invention relates to a method of determining the wet-cleaning interval for the interior walls of an RF-powered plasma processing chamber subject to unwanted material build-up.
2. Prior Art
Many thin film processes use plasma processes to facilitate the rapid and accurate fabrication of minute structures with desired properties. Plasma processes include the deposition and etching of insulators, conductors and semiconductors on a substrate, for example, a silicon wafer. The plasma process usually involves placing the substrate in a vacuum chamber, introducing process gases and applying radio-frequency (RF) power, typically 0.1 to 200 MHz, to create a plasma. The plasma consists of ions, electrons, radical gas species and neutral gas, all of which permit the desired reaction to proceed. The plasma reaction has many inputs, including RF power, gas type and flow rates, chamber pressure, substrate and wall temperatures, chamber wall conditions, electrode spacing, and so on.
The chamber configuration and chemistry used is chosen according to the desired process. For example, plasmas are used to etch dielectrics in semiconductor manufacture using specific plasma chamber designs such as Reactive Ion Etching (RIE) or Inductively Coupled Plasma (ICP) and using etching gases such as CHF3, CF4, O2 and so on.
In the etching of dielectric films, various by-products are created which may or may not be removed by the chamber pumping apparatus. By-products (including various polymers) which are not removed may be deposited as a series of layers on the walls of the reactor. The layers degrade over time, and eventually start to crack and flake, resulting in particle deposition on the wafer surface and decreased yield.
In the ideal production scenario, the plasma chamber operates continually, only stopping for scheduled maintenance. This maintenance often includes chamber cleaning using wet chemicals to remove the layers deposited on the chamber walls by the plasma process. This process is time consuming and can involve tool downtimes of several hours. If this wet-clean is performed too frequently, production uptime is negatively impacted and production costs are increased. On the other hand, if the deposited layer is not removed in a timely fashion, then product yield is negatively impacted by polymer flaking from the reactor walls.
Wet cleans can be scheduled according to a number of criteria. One method is to perform a wet clean after a given number of wafers are processed. A second is to use a measure of the time that RF power has been switched on in the chamber. A problem with these methods is that many production facilities manufacture different products with different process recipes or chemistries employed on each one. In this case, the relationship between wafer count or RF on-time and the incidence of flaking is not straightforward. Thus, wet-clean optimisation on wafer count or RF on-time is difficult. A third method is to use some sort of monitor for flaking events. One such method uses a particle measurement wafer to monitor for these flaking events. If particles are seen to increase dramatically then the chamber is taken down for a wet-clean. The particle measurement wafer will normally be run on a daily basis, so that yield loss of one day's production is possible. It would be far more advantageous to have a system that pre-empts the flaking event rather than observes the flaking event.
It is known that certain electrical signals derived from the plasma power source can be sensitive to many plasma processing events. The plasma represents a non-linear complex load in electrical terms. This results in the generation of harmonics of the RF driving signal. These harmonics, known as Fourier components, are very sensitive to changes both in the plasma process and the process parameters. It is generally accepted that monitoring the Fourier components of the RF power signal provides a useful way to monitor the plasma process. These components are a more direct measurement of the plasma process since they are more directly related to fundamental plasma parameters.
It is known to use an RF sensor to monitor and control RF plasmas by measuring the Fourier components of voltage and current. The sensor can be used in closed or open loop control, as for example, in etch end-point control or as in-situ monitoring of the plasma process. In either case the plasma can be terminated when one or more of the RF Fourier components reaches pre-determined limits.
U.S. Pat. No. 5,458,732 describes a method of determining the condition of a plasma-processing chamber by monitoring an RF signal at a frequency other than the fundamental frequency. For example, if the chamber wall conditions change, then signals in the harmonics of the RF applied signal can detect that change. The limitation of this approach is that the event is detected after it has occurred. It would be far more useful for the operators of plasma-processing chambers to have a precursor system for pre-empting flaking events. Another limitation of this technique as applied to chamber clean optimisation is that the harmonics are sensitive to many events on the plasma chamber and are also subject to gross changes following standard preventative maintenance. This means that the parameters of any control algorithm need constant updating. Changes in the process inputs will also change harmonics. Furthermore, harmonics vary widely from chamber to chamber, even for the same chamber type, so that any prediction algorithm is chamber dependent. Also, changes in substrate type, even the thickness of underlying layers, will change the harmonic signal.
There is a need, therefore, for an improved method of determining the wet-cleaning interval for the interior walls of an RF-powered plasma processing chamber, which allows the optimisation of chamber wet-clean cycles.